Recently-discovered carbon nanotubes are tubular materials having diameters of 1 μm or less. In the carbon nanotubes, ideally a hexagonal carbon network forms a tubular structure parallel to the axis of the tube, which may be a multiple tubular structure. The carbon nanotubes are composed of carbon, have different properties depending on the number of the hexagonal network and thickness of the tube, and are expected as future mechanical functional materials.
Dispersion liquids containing a solvent and a carbon nanotube uniformly dispersed therein are useful for producing the mechanical functional materials using the carbon nanotubes. For example, a nanocomposite containing a carbon nanotube uniformly dispersed in a polymer matrix can be produced by dissolving a polymer in a liquid containing a solvent and the carbon nanotube uniformly dispersed therein. Further, the dispersion liquids containing a solvent and a carbon nanotube uniformly dispersed therein can be used in optical instruments for their low scattering properties. Furthermore, purified dispersion liquids can be used for producing electronic devices such as transistors, electron emitters, and secondary batteries. For example, an emitter using fine carbon particles can be formed by the steps of dispersing the fine carbon particles in a solvent to prepare a suspension, forming a pattern of the suspension on a support of a substrate using a printing method such as casting, screen printing, or ink-jet method, and drying the solvent to obtain a desired shape.
It is generally known that water-soluble solvents, organic solvents, and mixtures thereof can be used for dispersing the carbon nanotubes. For example, it is disclosed that water, acidic solutions, alkaline solutions, alcohols, ethers, petroleum ethers, benzene, ethyl acetate, chloroform, isopropyl alcohol, ethanol, acetone, toluene, etc. can be used as the solvent (see JP-A-2000-72422).
However, methods for sufficiently dispersing the carbon nanotubes in the solvents have not been established. This is because the carbon nanotubes are formed into a bundle or cord due to the cohesive forces (van der Waals forces) between the carbon nanotube molecules. Further, the carbon nanotubes have smooth surfaces at atomic level, and the smooth surfaces causes reduction of affinity for the solvent. Thus, though the carbon nanotubes have specific, useful properties, it is extremely difficult to produce polymer-based nanocomposites, etc. with the carbon nanotubes uniformly dispersed, and this makes practical use of the carbon nanotubes difficult.
Though various methods for improving the dispersibility of the carbon nanotubes in the solvents have been attempted, the methods are not necessarily effective.
A method of dispersing a carbon nanotube in acetone under ultrasonic irradiation has been proposed (see JP-A-2000-86219). However, though the carbon nanotube can be dispersed under the ultrasonic irradiation, the carbon nanotube molecules start to aggregate after the ultrasonic irradiation, and the molecules are entangled when the carbon nanotube concentration is high.
Use of surfactants has also been proposed. A method of using a nonionic surfactant TERGITOL (trademark) NP7 under ultrasonication has been proposed. However, it is reported that, when the amount of the carbon nanotube is increased in the method, the carbon nanotube is aggregated, failing to obtain a uniform dispersion (see S. Cui, et al., Carbon, 41, 2003, 797-809). Further, a method for dispersing a single-walled nanotube in an aqueous solution of an anionic surfactant SDS, which has the step of subjecting the nanotube to ultrasonication in the aqueous solution, thereby adsorbing a hydrophobic surface of the nanotube to a hydrophobic portion of the surfactant to form an outer hydrophilic portion, has also reported (see Michael J. O'Connel, et al., SCIENCE, Vol. 297, 26, July 2002, 593-596). However, a water-soluble solvent is used in the method, whereby there is a limit in the application of the method. For example, when the method is used for polymer-based nanocomposites, only water-soluble polymers can be used. Though a method of adsorbing a hydrophobic portion of a water-soluble polymer PVP to the carbon nanotube surface instead of the surfactant has been proposed, also the method uses the water-soluble polymer and thereby has application limitations (see Michael J. O'Connel, et al., Chemical Physics Letters, 13, July 2001, 264-271).